The present invention relates to mode converters. it is particularly useful for TM.sub.01 TE.sub.01 conversion in a circular waveguide and in a type of biconical horn antenna radiating a horizontally polarised electric field.
For many future applications such as cellular radio at 60 GHz, omnidirectional antennas with horizontal polarisation are required. Since such uses are in mass markets, these antenna are preferably of simple construction so that they can be manufactured at a low price.
At a present biconical horn antennas are excited by the TE.sub.01 mode through a transverse slot in a circular waveguide. Problems arise in the excitation of the TE.sub.01 mode either by direct means or by way of a mode converter excited in a more simple mode, such as the TE.sub.10 mode in a rectangular waveguide. Moreover, as far as TM.sub.01 to TE.sub.01 mode converters are concerned, it is no easy to excite the TE.sub.01 mode in a circular waveguide and it is especially difficult for millimeter waves where dimensions are very small.